<template>
  <ul class="city-alpha">
    <li 
    class="item" 
    v-for="item in letters" 
    :key="item"
    :ref="item"
    @touchstart.prevent="start"
    @touchmove="move"
    @touchend="end"
    @click="handleLetterClick"
    >{{item}}</li>
  </ul>
</template>

<script>
  export default {
    name:'CityAlpha',
    props:{
      cities:Object
    },
    data(){
      return{
        touchStatus:false,
        startY:0,
        timer:null
      }
    },
    updated(){
      this.startY=this.$refs['A'][0].offsetTop
    },
    computed:{
      letters(){
        const letters=[]
        for(let i in this.cities){
          letters.push(i)
        }
        return letters
      }
    },
    methods:{
      handleLetterClick(e){
        this.$emit('change',e.target.innerText)
      },
      start(){
        this.touchStatus=true
      },
      move(e){
        if(this.touchStatus){
          if(this.timer){
            clearTimeout(this.timer)
          }
          this.timer=setTimeout(()=>{
            const touchY=e.touches[0].clientY-this.startY
            const index=Math.floor((touchY-this.startY)/20)
            if(index>=0 && index<this.letters.length){
              this.$emit('change',this.letters[index])
            }
          },16)
          
        }
      },
      end(){
        this.touchStatus=false
      }
    }
  }
</script>

<style lang="stylus" scoped>
@import '~css/mixins';
.city-alpha
  display flex
  flex-direction column
  justify-content center
  align-items center
  position absolute
  top 1.58rem
  right 0
  bottom 0
  width .4rem 
  .item
    line-height .4rem
    color $bgColor
</style>